In clinical nutrition we are used to dealing with the clinical management of patients, but of equal or greater importance is the study of how nutrition affects the development of diseases or modifies its manifestations. This field is complex and links physiological studies of nutrition to the epidemiological analyses which form the basis of thinking in public health in Mexico today. Thus a number of studies have investigated the nutritional risk factors leading to the development of diseases such as heart disease and cancer (1-10). This epidemiological research requires the difficult task of accurately assessing the food consumption of individuals: with poor methodologies the chances of erroneous results are very high. This has implications for both group and individual comparison. Physiological studies on the effects of highly controlled changes in food intake on risk factors then allows the epidemiology to be interpreted in metabolic terms. In this paper we illustrate some of the benefits of metabolic studies and some of the requirements for this successful conduct.